Measurements of Free-Stream Turbulence in some Low-Speed Tunnels at N.P.L.

Abstract

The r.m.s, fluctuation levels in the older low-speed tunnels at N.P.L. are found to be as follows: Compressed Air Tunnel, 0.3% rising to 0.6% at maximum speed and pressure; 13 ft x 9 ft Tunnel, 0.05 -> 0.2%; 9 ft x 7 ft (South) Tunnel, 0.18%; 18 in. Anemometer Tunnel, 0.2% in the unrestricted tunnel rising to 1% when running with diaphragms at low speed. Detailed measurements of the turbulence in the N.P.L. low-speed tunnels have not previously been made, and when information about the turbulence in the Compressed Air Tunnel was needed, the opportunity was taken to investigate some of the other tunnels as well. The results are presented here with a description of the tunnels and a short discussion of the likely sources of turbulence in each. Although intended chiefly for the information of those who use the tunnels it is hoped that the results will also be useful to tunnel designers. The authors wish to acknowledge the help of Mr. M. T. Gee, formerly of the Aerodynamics Division, N.P.L., in the planning and direction of the experiments. Messrs. R. J. Easton and K. W. Stuart assisted with the experimental work.